Tropical weather analysis - November 21, 2011
Tropical Storm Kenneth is strengthening. As of the latest NHC advisory, the following information was posted:
Wind: 50 mph, with higher gusts
Location: 12.2°N 107.0°W
Movement: WNW at 14 mph
Pressure: 1002 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
It is worth noting that Kenneth is the latest forming tropical storm in the Eastern Pacific basin since Hurricane Winnie in 1983 formed on December 4.
Kenneth has become considerably better organized over the last several hours. The satellite signature is impressive, with the center more involved with the central dense overcast than earlier. In addition, a recent SSMIS microwave overpass reveals something of an eye in the 85 GhZ channel, with a faint eyewall also evident to the south of this warm spot. In short, Kenneth is strengthening -- and quickly.
Figure 1. Latest infrared satellite image of Tropical Storm Kenneth, courtesy of NOAA's Satellite Services Division (SSD).
Conditions appear most favorable in the short-term, when Kenneth will traverse SSTs of 28-27C and be insulated from dry air. After about 36 hours, the SHIPS model shows the synoptic relative humidity values decreasing as Kenneth enters into a region of large scale subsidence. Despite the very favorable diffluent upper-level pattern forecast at that time, strengthening should be slower at that point. Given current trends, Kenneth is likely to become a hurricane within about 24 hours, and a brief period of rapid intensification is even possible if the inner core structure continues to improve. In about four days, Kenneth is forecast to encounter an increase in westerly shear, which along with sub-26C SSTs should induce weakening, possibly fairly rapid.
The track forecast may actually be harder than the intensity forecast. Go figure. Kenneth is currently being steered toward the west-northwest by a decaying mid-tropospheric ridge to its north. As the mid- to upper-level trough to the west moves north and fills, the ridge is forecast to briefly become reestablished, forcing the cyclone more westward in about a day or so. In about two days, a secondary perturbation within the westerlies is forecast to deamplify the ridge, which should once again cause Kenneth to turn back toward the west-northwest. This is where the divergence -- and thus uncertainty -- enters the fray.
Some of the models, such as the CMC, UKMET, NOGAPS and ECMWF, show a weaker Kenneth moving more westward in tandem with the low-level flow. Contrarily, the GFS and GFDL/HWRF pair show a deeper vortex moving more poleward in advance of a strong cold front. While the trough appears deep enough to impart at least some degree of northerly motion to Kenneth regardless of its exact strength, how strong Kenneth will be late in the period is uncertain. However, given the strong shear profiles forecast by the GFS at days four and five, I assume Kenneth will be weak enough to follow the former camp. However, if the model consensus continues to shift eastward, I will need to adjust my forecast as well.
I should probably note that both the GFDL and HWRF show Kenneth being pulled into Baja. This is considered highly unlikely.
A developing gale center located about 850 miles northeast of the northern Leeward Islands ("Invest 99L") is producing a large area of showers and thunderstorms to the north and east of the low-level center. Satellite and surface data indicate that the low-level center remains broad and very poorly-defined. Strong southwesterly shear associated with a nearby upper low is preventing convection from developing close to the center, even though the system is sitting underneath sea surface temperatures of about 27C.
Figure 2. Latest infrared satellite image of Invest 99L, courtesy of NOAA's Satellite Services Division (SSD).
Upper-level winds are only forecast to improve slightly over the next 48 hours, so any development will likely be subtropical given continued entanglement with the upper low. This is also supported by the global models.
This system poses no threat to land, and should eject out to sea under the influence of a cold front currently moving into the western Atlantic.